Our return voyage on the Marion Dufresne was very different from the voyage south. The first voyage was for logistics resupply, and delivery (and uplift) of personnel, plus we had twelve fare-paying tourists on board. The voyage back was an oceanographic survey voyage. There were a similar number of passengers on board (47), but more than half were oceanographers, with the remaining 20 being returning expeditioners from Crozet and Kerguelen.
Our next landfall after leaving Isles Kerguelen was refuelling at Mauritius, one day before disembarking on Reunion Island. But this was not an express trip. If we were to steam direct to Reunion, it would take about 5.5 days to cover the 3500 km from Kerguelen. Our trip took 14 days, as we zig-zagged from waypoint to waypoint in the southern Indian Ocean. Some of the monitoring programmes were continuous recording as the ship was in motion (e.g. multi-frequency sonar scanning of fish and plankton aggregations), while others required the ship to be held steady at a precise location for up to six hours at a time. There were many projects underway, and not all are described here. During this time we saw land only when we passed St Paul Island at dusk on 22 Jan, and later that night saw Amsterdam Island silhouetted against the night sky.
One of the long-term monitoring programmes was of plankton identification and abundance, using a continuous plankton recorder (CPR) towed behind the ship at a depth of 50 metres. This device is based on a design more than 100 years old, and works at towing speeds between 5 and 20 knots. Plankton enters the CPR through a tiny aperture, and is sandwiched between two strips of silk that feed over a set of internal spools at a rate proportional to the ship’s speed (regulated by a small external propeller). The silk is marked with ink in strips each representing 5 nautical miles (roughly 10 km), and the device is capable of capturing samples continuously for 500 nautical miles before it must be retrieved and new spools of silk ribbon installed.
There were two programmes that required the ship to be stationary for hours at a time (and for the CPR to be reeled in before we came to a stop). The first was hydrological surveying of a range of water parameters, requiring water samples to be collected at pre-selected depths down to a maximum of 4400 metres. The same sites (and depths) have been monitored annually since 1998, with the parameter of greatest interest for much of this time being the amount of carbon stored in water at different depths. Such long term research programs provide essential data to test models of human-induced climate change. A rack of bottles is gradually lowered to the sea floor (or some other predetermined depth), with each bottle programmed to close when at a certain depth. The bottles are then winched up to the surface again, with the whole process taking up to 6 hours for a deep sampling station.
The other programme that required the ship to cease moving was deploying or recovering whale monitoring hydrophones, which are left in place for a year. The devices were attached to a big lump of scrap metal on the seafloor (up to 5.2 km down) by a long tether (up to 4.2 km long). At the bottom of the tether is a release device, which can be ‘told’ to let go of the dead-weight by an acoustic signal from the ship. The hydrophone, data-storage unit and batteries are inside a large orange buoy at the top of the tether, but still more than a kilometre below the surface. When released, the buoy (with tether and release device trailing behind) rises at about 100 metres per minute, so it takes more than 10 mins to reach the surface, where it pops out like a breaching whale. Once the buoy is recovered, it takes more than 2 hours to winch in the longest tethers in order to retrieve the acoustically-triggered release device.
Deploying a new hydrophone is a similarly protracted process, as the buoy and tether are fed out first, then the dead-weight is dropped, and the release device (therefore the dead-weight also) is monitored acoustically on its slow descent to the sea-floor, so that its final resting spot can be recorded accurately for release and recovery of the hydrophone and data a year later. We could then move at normal speed to the next retrieval or deployment spot, but that might be in any direction of the compass, so that the net result over many hours was that we remained in much the same part of the southern Indian Ocean.
While we stooged around in uncharacteristically calm seas, a tropical cyclone (la tempête tropicale Corentin) developed to the north of us. With 14 metre-high waves forecast, the captain decided that it would be prudent to cancel the north-easternmost sampling station. We headed west, before setting a north-westerly course for Mauritius and Reunion, avoiding the worst of the storm.
The birds seen on our way north were similar to those recorded on the way south between Reunion and the Crozet Islands. As we approached St Paul and Amsterdam Islands, yellow-nosed mollymawks were common, and we saw several ‘dark-mantled’ sooty albatrosses. Many people on board were hoping to see an Amsterdam albatross – the world’s rarest albatross, with fewer than 30 pairs breeding per year, and only on Amsterdam Island. The only one that put in a close appearance did so when few people were on deck, and I did not have my camera at hand.
Once north (and west) of Amsterdam Island, great-winged petrels were the most numerous species, with the addition of a few flesh-footed shearwaters (which breed on St Paul Island, as well as back home in New Zealand). However, both the abundance and diversity of seabirds was soon to fall further. When we were 2-4 days out from Mauritius there were so few birds that I did not see any at all for more than 48 hours – though there were occasional flying fish, flying squid and one obliging sperm whale to provide some wildlife to look at. The only place I have ever been with fewer birds is the interior of the Antarctic continent. The near-complete absence of birds was in stark contrast to the extraordinary biomass supported by the waters of the Kerguelen shelf, which we had left in our wake only a week earlier.
Te Papa curator of vertebrates Dr Colin Miskelly’s participation in seabird research programmes on the French subantarctic island groups of Crozet and Kerguelen was at the invitation of Dr Charly Bost of the CEBC laboratory (Chizé) of CNRS (Centre National de la Recherché Scientifique), France, and was supported by the Institut Polar Français Paul Emile Victor (IPEV) and Te Papa.
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